It has been found in the manufacture of flexible polyurethane foams, and particularly in the manufacture of polyether foams employing the continuous casting method, that the interior of the foamed mass may exhibit a burned or scorched appearance. This scorching is produced by (1) the high temperature resulting from the exothermic foam-forming reaction, which is usually above 250.degree. F. and may be well over 300.degree. F.; (2) the retention of the heat at the central region of the foam mass due to the low thermal conductance of the foam; and (3) the initiation of an oxidation reaction as ambient air infiltrates the block following expulsion of the CO.sub.2 produced during the foam forming reaction. This oxidation reaction takes place with unreacted isocyanate and/or the hot foamed polymer and is undesirable for several reasons. Even at low levels, such oxidation can produce yellowing of light colored foams, which can make the foam product aesthetically unacceptable for some commercial uses. If the oxidation reaction is excessive, as evidenced by more than slight yellowing, a deleterious effect on foam properties will result. This undesired oxidation reaction can proceed with sufficient intensity to actually cause the foam to ignite and burn. The tendency towards this post-foaming oxidation reaction is greatest where the isocyanate index in the original formulation exceeds about 110, or when the rate of addition of polyol falls below the calculated value, as when there is a blockage in the polyol feed line or a pump failure. The tendency of the foam to scorch is also increased as the maximum exotherm is raised which increase can be caused by high levels of water in the formulation, e.g., above five parts per hundred of polyol.
The completion of the foam-forming polymerization reaction and the curing of the foam requires a finite period of time. Using commercial formulations well known to the art, the foam-forming reaction initially proceeds rapidly so that the foam gels or solidifies within about two minutes after the mixture is poured onto the casting surface. However, the maximum exotherm as determined by measuring the temperature at the interior of the foam block does not occur until about twenty to thirty minutes later. There is also a small proportion of remaining isocyanate which reacts over an even longer period of time, with additional cross-linking occurring as long as the temperature is above about 200.degree. F.
The evidence that oxidation is responsible for scorch or fire can be derived from the normal cure temperature curve for the foam block. After reaching the maximum exotherm, the foam mass begins to cool. Up to this point, the foam contains carbon dioxide, vaporized reactants and reaction by-products which are expelled from the exposed surfaces of the block. These vapors are readily apparent during manufacture, as they escape through the upper surface which is not contained by the release paper or film on which the foam is cast and which substantially eliminates the escape of gases from all but the upper surface of the block. As the foam mass cools and the volume of the gaseous reaction products on the interior of the block reduces, ambient air enters and permeates the interior of the block. This is the first time that a significant amount of air can enter the foam. It will be appreciated by those skilled in the art that the passage of a stream of heated air into a block of freshly manufactured foam could result in an acceleration of the oxidation reaction and the promotion of scorching, or even ignition of the foam mass. Thermal degradation of the foam interior without oxidation is possible, but unlikely. If the oxidation reaction generates heat more rapidly than the heat can be dissipated, then degradation will occur and can become a runaway reaction. Passage of air can actually promote or accelerate the oxidation reaction, at least until the air flow is sufficient to carry heat away more rapidly than it is being generated by the undesired reaction. Because low air flow rates, such as a localized draft, can increase the hazard of scorch or fire, there is a minimum effective flow rate that should be met to assure safe cooling.
The art has long recognized the desirability of a process which could be efficiently and cost effectively employed to rapidly cure freshly manufactured cellular polyurethane foam without interfering with the development of optimum physical properties. Among the many economies to be achieved would be the ability to reduce storage space and time, material handling and delays in processing orders from customers.
Furthermore, as environmental concerns are heightened, regulations prohibiting the discharge into the atmosphere of gaseous reaction by-products from the foam making process are becoming more common. Processes that provide for the capture and recovery of these compounds will be preferred, if not required in the future.
Various processes for treating flexible, air permeable cellular polyurethane foam to reduce the time required for curing the foam have been disclosed in the art.
A process for rapidly and uniformly cooling a freshly made section or mass of flexible, substantially open-cell polyurethane foam to improve its physical properties is disclosed in U.S. Pat. No. 3,890,414 which issued on Jun. 17, 1975. The disclosure of U.S. Pat. No. 3,890,414 is incorporated herein by reference. That process contemplates inducing a pressure drop across at least two opposing gas permeable surfaces of a hot, freshly polymerized, open-cell block of polyurethane foam to induce the passage through the block of a draft of cooling gas having an initial temperature of about 80.degree. F., or less. To the extent that this prior art process disclosed the initial moisture content, or relative humidity of the cooling gas stream that contacted the hot foam, it was well below the dew point or saturation level at the ambient temperature of about 75.degree. F. This prior art method also discloses the use of dry nitrogen gas, and of dry chilled air to effect the rapid cooling of the treated foam.
U.S. Pat. No. 3,061,885, issued Nov. 6, 1962, discloses a process for accelerating the rate of cure of substantially open-cell flexible cellular polyurethane foam employing the steps of crushing the foam material and impinging the surface of the foam with a gaseous stream of air from jets for from 2 to 10 minutes at a temperature of about 100.degree. to 250.degree. F. and a pressure of about 5 to 100 psi.
U.S. Pat. No. 4,537,912 issued Aug. 27, 1985, describes a process for the rapid post curing of porous blocks of polyether polyurethane foam utilizing a combination of humidified air and gaseous ammonia, primary or secondary amines to improve the compression set values of the cured foam. The foam blocks are subjected to this gaseous atmosphere containing ammonia, primary or secondary amines which is at a temperature ranging from 50.degree. to 150.degree. F. and a relative humidity of from 50% to 100%, and preferably at 70% relative humidity and about 115.degree. F. In one example, a block of polyether foam of unstated age was placed in an autoclave and live steam was introduced up to a pressure of twenty-one inches of mercury over a period of seven to ten minutes. [See col. 5, lines 52--65; col. 6, lines 3--13.]According to the inventors, this treatment had no beneficial effect on reducing the curing time for the foam sample.